Seedling identification - possibly piptanthus

The seedlings pictured, have been sitting doing nothing since emergence several weeks ago. Fed up waiting for a true leaf to emerge I investigated. The growth is going into small white tubers growing at the base of the stems, about an inch and a half below the surface.

 

If I remembered that I might not have to ask! Sown a year ago, label lost...

 

I've grown Piptanthus but cant remember what the seedlings look like, it was years ago.

But the bit about small white tubers doesn't seem to fit.

 

I found another of these seedlings yesterday in a nursery bed where last year I'd put a tray of Hardy Plant Society seed mix. Aha I thought, what about alstromeria - there's always alstromeria seeds in the HPS mix. But no, a look on the web put paid to that.